Defoliation-induced changes in carbon allocation and root soluble carbon concentration in field-grown Lolium perenne plants:: do they affect carbon availability, microbes and animal trophic groups in soil?

1. It is hypothesized that defoliation-induced changes in plant carbon allocation and root soluble C concentration modify rhizosphere C availability and, further, the abundance and activity of soil microbes and their grazers. To test this hypothesis, field-grown Lolium perenne swards were defoliated twice during their second growing season at two nitrogen availabilities (added N or no added N). Plant, soil and microbial attributes were measured 2 and 4 days after the last defoliation, and nematode abundance was measured 6 days after the last defoliation. 2. Defoliation decreased shoot production in plots where N was added, but had no significant effect in plots where N was not added. Root biomass and the ratio of root mass to shoot production were not affected. 3. Defoliation increased root soluble C concentration by 26% at the first harvest (2 days after defoliation) and by 18% at the second harvest (4 days after defoliation). Leaf N concentration was 27% lower in defoliated than in non-defoliated swards at the first harvest, while that of stems was 14% higher in defoliated swards at both harvests, and that of roots was not affected. Defoliation increased root C : N ratio, decreased stem C : N ratio, and did not have a statistically significant effect on leaf C : N ratio. 4. Soil attributes (soil soluble C concentration and soil C and N concentrations) were not affected by defoliation. Similarly, microbial attributes such as microbial C and N content, bacterial abundance in rhizosphere soil, and diversity of C sources utilized by the rhizosphere microbial community, did not differ between defoliated and non-defoliated swards. 5. Among nematode trophic groups, defoliation reduced the abundance of fungivorous and herbivorous nematodes by 70 and 47%, respectively, but did not affect the abundance of bacterivorous, omnivorous and predatory nematodes. 6. Although defoliation altered plant C allocation and root soluble C concentration, these changes did not influence C availability, soil microbial growth or the abundance of bacteria-feeding nematodes in the plant rhizosphere. Instead, the effects on root- and fungus-feeding nematodes suggest that the effects of defoliation on soil communities were propagated not through the effects of root-released C on bacteria and bacterial grazers, but through effects of root quality on root-feeders and possibly through effects of mycorrhizal fungi on fungus-feeders.